This application claims priority under 35 USC § 119 to Korean Patent Application No. 2004-56237, filed on Jul. 20, 2004, the contents of which are herein incorporated by reference in their entirety for all purposes.
1. Field of the Invention
The present invention relates generally to tuning radio frequency signals, and more particularly to a double conversion RF tuner with reduced bandwidth for a local oscillator.
2. Description of the Related Art
A tuner generally amplifies a radio frequency (RF) signal received by an antenna and converts the received RF signal into a desired intermediate frequency (IF) signal. Tuners are used in a wide variety of applications such as televisions, cellular telephones, cable modems, etc. For example, cable boxes are equipped with a tuner to select from numerous cable channels that are received in a frequency band extending from about 54 MHz and to about 862 MHz.
The receiving schemes of the related art may be largely classified into a homodyne scheme and a heterodyne scheme. The homodyne scheme is referred to as “direct conversion” and directly converts the RF signal into a baseband signal. Thus, the term “direct conversion” indicates that an RF frequency is directly converted into a baseband frequency without the conversion into an intermediate frequency (IF). In contrast, the heterodyne scheme uses the intermediate frequency (IF) signal having a lower frequency than the radio frequency (RF) signal. The heterodyne scheme has advantages that amplification may be easily performed in a reception/transmission system and selectivity and stability may be high.
In the heterodyne scheme, two types of tuners known as ‘single conversion’ and ‘double conversion’ tuners are used. In the single conversion tuner, the RF signal corresponding to a selected channel is converted to the IF signal. The double conversion tuner, on the other hand, converts the RF signal corresponding to a selected channel to a first IF signal and then converts the first IF signal into a second IF signal (desired final IF signal). The ability of the double conversion tuner to handle larger amounts of signals has resulted in the widespread use of double conversion tuners.
The double conversion tuner uses a so-called double conversion architecture (or up-down conversion architecture). The double conversion architecture has up and down conversion stages to ultimately down convert an input RF signal to the desired final IF signal. The input RF signal is first up converted to the first intermediate frequency having a higher frequency than the RF signal in the up conversion stage. The first intermediate frequency is then down converted to the desired final IF signal having a lower frequency than the RF signal in the down conversion stage.
However, the double conversion tuner has disadvantages that a frequency tuning bandwidth of a local oscillator (i.e., bandwidth of a local oscillator) is too wide for one voltage controlled oscillator (VCO) to cover numerous channels in the input RF signal. FIG. 1 is a block diagram illustrating a conventional double conversion RF tuner. Referring to FIG. 1, the conventional RF tuner includes an antenna 100, a first filter 110, a tuning section 120, a second filter 130, a third filter 140, and an analog-to-digital converter (A/D) 150.
For example, the input RF signal is to be up converted to the first intermediate frequency IF1 of about 1220 MHz, while the input RF signal is to be ultimately down converted to the second intermediate frequency IF2 of about 44 MHz. When a high-side local oscillator (HSLO) is used, a local oscillator frequency FLO1 provided from a local oscillator 123 of the tuning section 120 has a frequency range of from about 1274 MHz to about 2082 MHz (to select a down conversion result of a mixer 124) so that the input RF signal may be up converted to the first intermediate frequency IF1.
Additionally, using the high-side local oscillator (HSLO), a local oscillator frequency FLO2 provided from a local oscillator 127 of the tuning section 120 has a frequency of about 1264 MHz (to select a down conversion result of a mixer 128) so that the first intermediate frequency IF1 may be down converted to the second intermediate frequency IF2. (Herein, the local oscillator frequencies FLO1 and FLO2 may vary depending on whether an up conversion result or a down conversion result of the mixer is chosen by a receiver designer).
Thus, the frequency range (or frequency bandwidth) of the local oscillator 123 is too wide for one voltage controlled oscillator (VCO) so that two or more VCOs are needed for the conventional double conversion RF tuner.